The manufacture of reinforcing plies intended for the construction of tires is widely known in the prior art and involves taking, from a continuous ply known as a straight ply, at a given cutting angle b, lengths of ply of a given physical width and hereinafter known as widths. The straight ply is made up of threads coated in a rubber compound, which are mutually parallel and oriented in the longitudinal direction of the ply. The widths are placed end to end to constitute a continuous reinforcing strip. The reinforcing threads make an angle b with the longitudinal direction of the reinforcing strip. This angle b is non-zero and smaller than 90°. The value of the angle b is generally comprised between 15° and 70°.
A second step of the tire-building process involves taking lengths of reinforcing ply, or reinforcing ply from the reinforcing strip. The physical length of these lengths of reinforcing ply is tailored to suit the circumference for the laying of the said reinforcing ply on the reinforcing ply building means such as a crown form, which is generally cylindrical.
The lengths of reinforcing ply are cut between two reinforcing threads. Hence, prior to laying, the reinforcing ply is in the form of a parallelogram the front and rear selvedges of which make an angle b with the longitudinal direction of the reinforcing ply and the lateral selvedges of which are parallel to this same direction.
Particular attention is paid to maintaining the geometric characteristics of the reinforcing ply throughout the process, from the cutting-to-length step to final transfer onto the tire-building form. This is because it is necessary as far as possible to avoid any deformation that could alter the length of the reinforcing ply or change the angle of the threads, particularly where there is a desire to automate the entire process.
For this reason, it is known practice, when the reinforcing threads have magnetic properties, for the cut-to-length reinforcing plies to be transferred using magnetic belts capable of depriving the reinforcing ply of any unwanted movement while it is being transferred from the cutting station to the crown form.
It may also prove advantageous for the cut-to-length reinforcing ply to be transferred from a first transfer belt to a second transfer belt dedicated to the function of bringing the reinforcing ply up close to and laying it on the tire-building form.
The two belts then overlap over a certain length and are distant from one another by as small a distance as possible, but which distance is greater than the thickness of the reinforcing ply, so that the reinforcing ply can be transferred from the first belt to the second belt with the least possible unwanted movement of the said plies.
By way of example, publications U.S. Pat. No. 4,411,724 or, alternatively, U.S. Pat. No. 4,769,104, describe transfer belts of this type, in which the force of attraction of the electromagnetic means of the second belt is greater than the force of attraction of the magnetic means of the first belt, allowing a metal ply to be transferred from the first belt to the second belt. The magnetized regions of the first and of the second belts overlap.
However, in the case of reinforcing plies the geometry of which can be likened to that of a non-rectangular parallelogram, it is difficult to control the transfer of the rearmost tip of the reinforcing ply onto the second belt without altering the position of the forward tip of the next reinforcing ply which is intended to remain on the first belt. The problem here is that the consecutive plies downstream of the cutting tool are contiguous at the rear part of the front reinforcing ply or first reinforcing ply, and at the front part of the next reinforcing ply or second reinforcing ply.
Because the shape of transfer belts is generally that of a rectangle, it is difficult to maintain the magnetization under the forward tip of the second reinforcing ply while at the same time releasing this same magnetization under the rear tip of the first reinforcing ply so that the said rear tip of the first reinforcing ply can be transferred from the first belt to the second belt.